use Graphics::Magick;
$image = Graphics::Magick->new;
$image->Set(size=>'10x10');
$image->ReadImage('xc:white');

$image->Draw(stroke=>'red', primitive=>'rectangle', points=>'3,3 7,7')
+;
$image->ColorFloodfill(geometry=>geometry, x=>4, y =>4 , fill=>'blue',
+ bordercolor=>'red');
#$image->Set('pixel[1,3]'=>'green');
$image->Write('block_normal.png');
[download]

(Just) Horizontal and vertical lines are pretty easy to deal with.

On the other hand, if the OPs images contain diagonals, it gets much harder. Unless they are all the same angle, or just a few angles.

Of course, his images might contain curves, which gets harder still.

And if they contain a mix of all three ...

And, then there is black and white; or grey scale or 8-bit dithered; or 24-bit fullcolor; or 48-bit; plus or minus alpha channels ....

Attempting a totally generic solution is a waste of effort if his images only contain horizontal and vertical lines.

Posting a solution for horizontal and vertical lines is a waste of time, if all his lines are curves.

Knowing what you're up against is the key to an efficient, effective algorithm; and a picture paints a thousand words.

---

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority". I knew I was on the right track :)

In the absence of evidence, opinion is indistinguishable from prejudice.

The pictures essentially two colours with black lines on a white background. I want to thin the black lines. These black lines are at all angles and may be curved. I can easily make an image available but I am not sure where to put/send it to.